Here is an excerpt from a 1939 paper that describes a randomly operated (as opposed to Geiger-coincident triggered) expansion cloud chamber with some track photographs.
Description of Cloud Chamber
The essential parts of the cloud chamber used in the present work are shown in the figure below:
The main compartment A, where the tracks are formed, is cylindrical, about 30 cm in diameter and 30 cm long. Before an expansion is made the rubber diaphragm B is close to the perforated brass plate C, the valve D is shut and the reservoir E partially evacuated. To operate an expansion the catch F is knocked in the direction of the arrow. This opens the valve, and the rubber diaphragm falls on to the perforated brass plate G.
Most of the photographs were taken with ethyl alcohol vapour as condensant, the expansion ratio for which is about 1-16.
The side of C facing A is covered with black velvet, which prevents irregular motion of the gas in A during an expansion and provides a dark background for the tracks. The camera used had lenses of focal length, f, of 12 cm, and an aperture of f/5.8. It was focused on the plane J between the magnetic field coils H. With a magnification of 0.28, such as was chosen, the effective depth of focus was about 5 cm.
To avoid distortion of the tracks by convection currents it is very important that the walls of the chamber be kept at a uniform temperature (cf. Blackett and Wilson 1937), a variation of half a degree or so being the limit tolerable.
In view of this the brass formers containing the field coils (the inside of which are within 2 cm. of the chamber walls) were kept at room temperature by passing water through the copper tubes K soldered to the side of the formers. With this precaution the distortion of the tracks (the estimate of which is described in the next section on the spectrum) corresponded in general to a radius of curvature (in the chamber) greater than 7m, i.e. to a dip in an arc 10 cm long of less than 0.18 mm. Convection currents set up as the result of the expansion are confined, during the sensitive time of the chamber, to a region close to the walls and do not give any trouble.
A mechanical method of timing the various events in the operation of the chamber proved quite satisfactory. With the arrangement used the timing could be reproduced at long intervals with an accuracy of at least 0.01 sec.
This reproducibility is important, especially when the chamber is used for determining decay periods of long-lived radioactive substances, and it is doubtful if an electrical method of timing would be equally reliable. The chamber described here has been used for this purpose, the radioactive substance being mounted inside the chamber as shown in the figure (Walke, Williams and Evans 1939).
The interval of time after an expansion during which the supersaturation remains sufficient to cause condensation on ions—the sensitive time of the chamber—was determined by counting the tracks from a radioactive source which was uncovered at different times after the expansion, the expansion ratio used being the maximum consistent with not giving a general cloud. It was found to be about 0.4 sec, which is about 20 times greater than the sensitive time of an ordinary sized chamber about 4 cm deep.
Track Photographs
Figure 3: a and b represent an electron pair. Energy of a = 90 MeV b = 24 MeV. c is probably an electron track with energy 8 MeV not associated with a and b. e is probably a mesotron track with energy in the order of GeV. d and f are "old" tracks, probably of mesotrons with energy exceeding 600 MeV. (Magnetic flux density, H = 2200 G)
Figure 4: a and b represent an electron pair (energies 50 MeV and 70 MeV). d is probably an unassociated electron with energy 70 MeV.
Figure 5: a is probably a mesotron track with energy exceeding 600 MeV, and b an electron track produced by a (energy = 15 MeV). c is an independent track, probably of a mesotron of 400 MeV. (H = 1000 G)
Figure 6: a, b, c, d, e, f, g are nearly parallel electron tracks, probably representing a portion of a more extensive shower. Energies exceeding 100 MeV. (H = 600 G)
Figure 7: Shower of 8 tracks. 3 have energy < 30 MeV, and 5 energy > 30 MeV. Gas is a mixture of hydrogen and air. (H = 200 G)
Figure 8: An old shower of 7 tracks. 5 are positive electrons with energy less than 100 MeV. 2 have energy greater than 100 MeV, sign of charge uncertain. (H = 2200 G)
REFERENCE
E. J. Williams, "Some Observations on Cosmic Rays Using a Large Randomly Operated Cloud Chamber", Proc. Roy. Soc. A, Vol. 172, No. 949, pp. 194-212, (1939)
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